Light output uniformity is one of the most important parameters for measuring qualities of a backlight source. Usually, a central point of the backlight source has the highest luminance according to requirements of optical qualities. For an edge-type backlight source, the center having the highest luminance will necessarily cause the luminance on a side remote from the light source to be reduced, thereby forming a dark region in a central part on the side remote from the light source. Even if mesh-point structures are distributed most densely in such a dark region, the optical luminance still cannot be improved.